1. Field of the Invention
The present invention relates to an alkaline battery separator, i.e., a separator for an alkaline battery, and a process for producing the same. In the specification, the xe2x80x9calkaline battery separatorxe2x80x9d will be sometimes referred to as the xe2x80x9cseparatorxe2x80x9d.
The alkaline battery separator of the present invention may be used in, for example, an alkaline primary battery, such as an alkaline-manganese battery, a mercury battery, a silver oxide battery, or an air battery, or an alkaline secondary battery, such as a nickel-cadmium battery, a silver-zinc battery, a silver-cadmium battery, a nickel-zinc battery, a nickel-hydrogen battery or a chargeable alkaline-manganese battery. The separator of the present invention is preferably used in the nickel-cadmium or nickel-hydrogen battery.
2. Description of the Related Art
In an alkaline battery, a separator is installed between a positive electrode and a negative electrode to separate them from each other and prevent a short-circuit, and to hold an electrolyte and thereby smoothly carry out an electromotive reaction. The separator should hold the electrolyte, such as potassium hydroxide, and is preferably made of polyolefin-based fibers having a high alkali resistance.
However, polyolefin-based fiber has a poor affinity for an electrolyte, and thus a separator of the polyolefin-based fibers has a disadvantage in having an insufficient capacity for holding an electrolyte (i.e., the electrolyte-holding capacity). Therefore, a fiber sheet of the polyolefin-based fibers is subjected to various treatments for imparting a hydrophilic property, and thus improving the electrolyte-holding capacity.
A graft-polymerization of vinyl monomers is known as one of the treatments for imparting the hydrophilic property. For example, the Japanese Publication of International Application (Tokuhyo) No. 6-509208 discloses a separator prepared by graft-polymerizing vinyl monomers to a fabric composed of polyolefin-based fibers. This publication also discloses a process for manufacturing the separator, comprising the steps of impregnating the fabric of polyolefin-based fibers with a solution of vinyl monomers, and irradiating the fabric with ultraviolet light under conditions such that the fabric is not exposed to oxygen.
When the resulting separator is installed in a nickel-cadmium or nickel-hydrogen battery or the like, however, deterioration occurs due to the action of oxygen generated upon overcharging, and a short-circuit is caused. Further, an electrode is affected by a product from the deterioration caused by oxygen, and the lifetime of the battery is shortened. Furthermore, the effect for inhibiting a self-discharge is poor.
The present inventors engaged in research into obtaining an alkaline battery separator without the above disadvantages, and as a result, found that when a fiber sheet used as the separator carries, on outer surfaces of fibers forming the surface of the fiber sheet, a substance having a peak of a bond energy in a particular range, an alkaline battery separator not susceptible to oxygen generated in the battery and exhibiting a high oxidation resistance and an excellent self-discharge inhibition effect can be obtained. Further, the present inventors found that when, in a fiber sheet used as the separator, the amount of carboxyl groups bonded to surfaces of fibers forming the inside of the fiber sheet is larger than that of carboxyl groups bonded to outer surfaces of fibers forming the surface of the fiber sheet, an alkaline battery separator not susceptible to oxygen generated in the battery and exhibiting a high oxidation resistance and an excellent self-discharge inhibition effect can be obtained. In addition, the present inventors found that a desirable fiber sheet can be prepared by a first graft-polymerization in the presence of oxygen, and a second graft-polymerization under the condition that the fiber sheet is surrounded with a gas-nonpermeable film.
Further, the present inventors engaged in research into obtaining an alkaline battery separator exhibiting an excellent self-discharge inhibition effect, and as a result, found that when a fiber sheet used as the separator entraps a large amount of ammonia, an alkaline battery separator exhibiting a very excellent self-discharge inhibition effect can be obtained, and that the fiber sheet capable of entrapping a large amount of ammonia can be prepared by, for example, carrying out the first and second graft-polymerization procedures, using a graft-polymerizing liquid containing a surface active agent. When the resulting fiber sheet is used as an alkaline battery separator, not only is the self-discharge inhibitory effect is very excellent, but also an excellent wettability is obtained. Further, the resulting batteries show a good consistency in the rate of capacity maintained. The present invention is based on the above findings.
Accordingly, the object of the present invention is to provide an alkaline battery separator not susceptible to oxygen generated in the battery and exhibiting an excellent oxidation resistance and an excellent self-discharge inhibition effect, and a process for producing the same.
Another object of the present invention is to provide an alkaline battery separator exhibiting an excellent self-discharge inhibition effect, particularly an alkaline battery separator exhibiting an excellent self-discharge inhibition effect, an excellent wettability, and an excellent consistency in the rate of capacity maintained, and a process for producing the same.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, there is provided an alkaline battery separator comprising a fiber sheet containing, on outer surfaces of fibers which form a surface of the fiber sheet, a substance having a peak of a bond energy at 530.5 to 531.5 eV (electron volt) as measured by an X-ray photoelectron spectrometer at a photoelectron-taking-off angle of about 30xc2x0. Hereinafter, the fiber sheet of this embodiment will be sometimes referred to as a surface-oxidized fiber sheet.
Further, in accordance with the present invention, there is provided an
alkaline battery separator comprising a fiber sheet containing a larger amount of carboxyl groups bonded to and present in surfaces of fibers forming an inside of the fiber sheet than the amount of carboxyl groups bonded to and present in outer surfaces of fibers forming a surface of the fiber sheet. Hereinafter, the fiber sheet of this embodiment will be sometimes referred to as an internally highly-carboxylated fiber sheet.
Further, in accordance with the present invention, there is provided an alkaline battery separator comprising a fiber sheet capable of entrapping ammonia in an amount of 0.4 mmol/g or more in average. Hereinafter, the fiber sheet of this embodiment will be sometimes referred to as a high ammonia-entrapping fiber sheet.
An xe2x80x9caverage amount of ammonia entrappedxe2x80x9d as used herein means an average of values measured at 10 or more points randomly selected, for an amount of entrapped ammonia. The xe2x80x9caverage amount of ammonia entrappedxe2x80x9d can be obtained by measuring an amount of entrapped ammonia at 10 or more points randomly selected in a fiber sheet to be examined, according to a process mentioned below, and calculating the average.
In the present specification, a fiber sheet having at least one of the properties of the surface-oxidized fiber sheet, the internally highly-carboxylated fiber sheet and the high ammonia-entrapping fiber sheet, or combination thereof, will be sometimes collectively referred to as a highly functional fiber sheet.
Further, in accordance with the present invention, there is provided a process for producing an alkaline battery separator comprising the steps of
subjecting a fiber sheet to which graft-polymerizing liquid containing graft-polymerizable monomers, oligomers, and/or polymers are attached, to a first graft-polymerizing treatment in the presence of oxygen, and then
subjecting the fiber sheet to a second graft-polymerizing treatment under the condition that the fiber sheet is surrounded with a gas-nonpermeable film, to thereby obtain a graft-polymerized fiber sheet for the alkaline battery separator.
Further, in accordance with the present invention, there is provided a process for producing an alkaline battery separator comprising the steps of
subjecting a fiber sheet to which a graft-polymerizing liquid containing graft-polymerizable monomers, oligomers, and/or polymers and 0.01 to 3 mass % of a surface active agent is attached, to a first graft-polymerizing treatment in the presence of oxygen, and then
subjecting the fiber sheet a second graft-polymerizing treatment under the condition that both sides of the fiber sheet are fully covered with a gas-nonpermeable film, to thereby obtain a graft-polymerized fiber sheet for the alkaline battery separator.